**Introduction:**
In the field study on grass, a variety of grass with viable pollen grains from its bisexual flowers was placed on the healthy stigma of the same flower, but there was no fertilization. However, when similar pollen grains from another flower of the same variety were placed on the stigma, the flower fertilized. This phenomenon can be explained by the outbreeding device present in the grass variety.

**Outbreeding Device:**
The outbreeding device is a mechanism employed by plants to prevent self-fertilization and promote cross-pollination. It ensures genetic diversity by facilitating the exchange of genetic material between different individuals of the same species.

**Explanation:**
1. **Self-Incompatibility:** The grass variety in question likely possesses a self-incompatibility mechanism. This mechanism prevents self-fertilization by inhibiting the growth of the pollen tube or preventing the fusion of male and female gametes within the same flower.

2. **Bisexual Flowers:** The grass variety has bisexual flowers, which means that each flower possesses both male (stamens) and female (pistil) reproductive organs. This allows for the possibility of both self-pollination and outbreeding.

3. **Viable Pollen Grains:** The pollen grains produced by the bisexual flowers of the grass variety are viable, meaning they are capable of fertilizing the female reproductive organs.

4. **No Fertilization with Same Flower's Pollen:** When the viable pollen grains from the bisexual flowers were placed on the healthy stigma of the same flower, no fertilization occurred. This can be attributed to the self-incompatibility mechanism, which prevents self-fertilization within the same flower.

5. **Fertilization with Pollen from Another Flower:** In contrast, when similar pollen grains from another flower of the same variety were placed on the stigma, fertilization occurred. This suggests that the grass variety has a mechanism that allows for outbreeding, promoting cross-pollination and genetic diversity.

6. **Outbreeding Advantages:** Outbreeding offers several advantages to plants, including increased genetic variation, improved adaptability, and enhanced resistance to diseases and environmental changes. By promoting cross-pollination, the grass variety can ensure the survival and success of its offspring.

**Conclusion:**
The field study on grass demonstrates the presence of an outbreeding device in the grass variety, which prevents self-fertilization within the same flower but promotes fertilization through cross-pollination with pollen from another flower. This mechanism helps to maintain genetic diversity and improve the adaptability and survival of the grass variety.